It's a recursive problem you'd basically be talking to nobody (and you wouldn't be reading this comment on your writing either).
After all what's to stop those 'above' you from setting the clock back even further.
Eventually only those people not using that feature will be able to communicate, which puts you back at square one.
Presuming this was done where each person had fine control over the date, you may be right. However, suppose each person only had a single binary control, essentially "hide everyone newer than me."
Furthermore, rather than "newer than me" being defined by a join date specific to each user, instead it would be defined by the start date specific to a particular generation of users (the news.yc folks do seem to track mass user migrations). Visually (bars are new.yc join floods, time moves to the right):
time: |--------|---|------|-----|---...
group: 1 2 3 4 5
This would still provide each user with the ability to view the site more-or-less as it was upon first arrival, without actually allowing each user fine granularity in locking everyone out.
(for the record, such a feature isn't as useful as simply applying social & administrative pressure to keep quality up; I just wanted to mention an alternative implementation that doesn't turn into an arms race)
Maybe turning off new signups whenever some popular blog mentions Hacker News might raise the bar somewhat. That would get only those in who find HN interesting enough to bookmark it until the flood drops off.
Your approach involves freezing the user base so that nobody new can join. Granted, it's an innovative take on it I've never seen before, but that's still what it is. The problem with all such approaches is that a certain percentage of the user base wanders off over time, so your frozen userbase constantly shrinks. Eventually it gets below the point you care. (Or, in most cases, below the critical mass of the community.) It only works correctly in the static case where nobody ever leaves.
In your case you could raise the threshold, but all that does is expose you to the very things you were trying to get away from. If there was a correlation between "the people who stuck around" and "quality posters" that might work, but I doubt such correlations exist. (Then again, I don't know, as my words imply.)
About two years ago, I scraped a single article with over a thousand comments on Slashdot and looked at all the UIDs. I theorized that all the old folk would be gone. Instead what I found is that the UIDs were fairly uniformly distributed across the entire base, which, if you think about the temporal distribution of people joining, is actually a moderately interesting result. I just eyeballed the list of unique UIDs, I didn't actually plot it out, but it wasn't what I was expecting. Some small core stick around and keep posting, but over time the vast bulk of the early group leave.
They'd have to, there are just too many reasons to leave and only a very limited few to stick around indefinitely.
Eventually everybody dies, gets a life or lands in some sort of crisis in their lives that changes it to the point where spending time on social sites is just too expensive.